This invention relates to an interrogator-responsor system for secondary radar devices with friend-foe recognition in which the answering signals which are received in response to an interrogation are investigated as to whether they are coded in a definite manner and are different from the expected coding of signals, such as signals which are being identified as coming from foe-responsor devices.
In coded friend-foe recognition on the basis of secondary radar it is the primary purpose to prevent a hostile object from presenting itself as a friendly object. By impulse controlled code changes and other safety measures a foe has no opportunity to produce for its own use codes correctly timed with sufficient probability. For simulating a friend recognition, a foe is required to listen constantly to the answers of friends in order to possess the true recognition existing at any moment. If an enemy object has the possibility of receiving, briefly storing and transmitting responses upon inquiry, as long as the same responses are valid, then a genuine "friend" cannot be distinguished from a "counterfeit" friend, when the hostile object is reached by the respective response later than a friendly object which is in the neighborhood and has the benefit of the previous interrogations and therefore permits a repetition of the valid response for a sure deception. In order to distinguish a "genuine" friend from a "counterfeit" friend in spite of the foregoing, and in order to exclude other disturbing possibilities, the present invention proposes particular steps.